A measurement of specific absorption rate typically involves a three-dimensional characterization of an electromagnetic field in near-field conditions. The electromagnetic field penetrates a three-dimensional object such as, for example, a mannequin filled with a medium simulating a biological tissue. The electromagnetic field has an amplitude that can vary considerably from one point to another within the object. In order to determine the specific absorption rate, the amplitude of the electromagnetic field must be determined at many points in the object. A state-of-the-art approach consists in using a basic probe that comprises a single antenna device, which is arranged to measure three orthogonal components (X, Y, and Z) of the electromagnetic field. The basic probe is moved within the object so that the basic probe is successively located at various points. The basic probe then measures the amplitude of the electromagnetic field at these various points. However, this is a relatively slow process.
The patent publication WO 2004/079299 describes a method of measuring a specific absorption rate (SAR) in a phantom filled with a liquid, which reconstitutes the dielectric properties of a biological tissue. The phantom is exposed to a microwave emission from an antenna. The amplitude and phase of the electric field inside the phantom is measured for a plurality of points on a given surface, which is defined in a concentration near-field zone of the electric field. A near-field near-field transformation from the data measured on the surface is performed so as to determine the electric field in the volume inside the phantom. The value of the SAR is then calculated.
In more detail, the aforementioned patent publication describes a network of probes formed by a squared thin rigid substrate having sides of 70 mm and carrying 36 bipolarized probes. Each probe is formed by 4 separate strips etched in cross on a printed circuit substrate. Each strip of each dipole is connected to the central core of a thin vertical coaxial cable. Thus, two cables are connected to the two strips of the same dipole. These cables are connected to the terminals of a balun device. A bipolarized probe thus requires two separate baluns and four coaxial cables. The four coaxial cables are coupled to each other to ensure contact between the shields and so that the four strips are close without touching each other.